The invention relates to using multicarrier modulation navigation with at least one signal of opportunity.
The Global Positioning System (GPS) generally provides worldwide high-accuracy positioning. However, GPS requires lines of sight to multiple satellites and may be blocked or jammed, hence backup navigation techniques are of interest. Navigation via signals of opportunity uses existing radio infrastructure as ad hoc navigational beacons. A mobile receiver determines its position by computing a time difference of arrival (TDOA) between signal reception time at the mobile and at a reference receiver. The drawback is that this requires communication between the reference and mobile receivers.
Multicarrier modulation may be an ideal candidate for a navigation signal of opportunity. Communication between the reference and mobile may be very minimal for multicarrier modulation compared to other modulation types, since the block structure inherently aids synchronization of the two received signals.
Measurements that can be taken from signals of opportunity include the angle of arrival, the received power level, or the time difference of arrival (TDOA) at multiple receivers. TDOA may be preferable since it is difficult to get sufficient position accuracy from angle or power measurements. One difficulty encountered with using TDOA is that for each TDOA measurement, there must be either two transmitters sending the same signal or two spatially separated receivers measuring the same transmission. Usually only one transmitter is available for each signal of opportunity, hence a “reference receiver” may be placed at a known location, and the mobile (whose position is to be determined) must cooperate with the reference receiver. TDOA measurements generally may require some form of correlation between the two received signals.
One TDOA system is Long Range Navigation (LORAN). LORAN Operates in the low frequency band at 90 to 110 kHz. In this system, multiple synchronized transmitters radiate pulses of radio frequency energy. An airborne or shipborne receiver measures the TOGA of the pulses from the different transmitters. Each measured TDOA defines a hyperbolic line of position for the receiver. The intersection of multiple lines of position may be used to supply the location of a mobile receiver, as shown in FIG. 1a. 
NAVSYS Corporation designed a navigation system which lessens the effects of GPS jamming or interference. The system uses multiple receivers at known locations to determine the position of a single transmission source; e.g. a GPS jamming device. Once the GPS interference source location is known, it can then be neutralized.
The present invention differs from these two systems. LORAN uses designated transmitters with a signal built for location identification, not signals of opportunity. The signal markers are built into the transmission in LORAN unlike the present system which uses a cyclic prefix in certain digital signals for distance calculations using block boundaries. The present invention in also uniquely computationally efficient and therefore “fast enough to be useful.” The present invention is also designed to consume a small amount of the available bandwidth and therefore can operate in cooperation with the signal provider, rather than usurping the original intent of the signal.
NAVSYS is designed to locate GPS jamming with the same designated transmitters and signal limitations as observed with the LORAN system. It does not mitigate GPS outages.